1. Field of the Invention
The present invention relates to the use of voltage sensitive threshold conduction devices for providing a motor speed/brake controller, and more particularly, in accordance with a first aspect of the invention, the use of such devices to provide a speed-sensitive brake portion for a DC motor controller. By connecting one or more of the devices across the windings of the motor, a variable setting of the speed at which the braking function ends is provided, thereby allowing the motor to be braked at speeds above a set point and to "coast" at speeds which are lower. In accordance with a second aspect of the invention, a series connection of a plurality of pairs of such devices, each pair having oppositely poled voltage threshold polarity, are selectively connected so as to be inserted between a source of power and the motor for providing the speed control portion of the controller. Such controllers are particularly useful for model (slot) cars.
2. Description of the Prior Art
Currently, DC motors such as those used in models commonly employ a variable resistor to control motor speed. This method is both simple to construct and operate and is functional. As shown in FIG. 1, a wiper 2 moves across the exposed windings of a resistor portion 4 to produce a variable resistor 6. Variable resistor 6 is connected in a series circuit with a DC motor 10 of a model and a power source 8 (via the metal tracks of a model racing system, not shown). The speed of the motor varies in accordance with the voltage divider created by the combination of the resistance of the DC motor and that portion of the variable resistor which is caused to be in the series circuit by the action of the wiper 2. Variable braking, when used, is accomplished by shunting the DC motor 10 with a portion of another variable resistor 12. Here, the variable resistor 12 operates in the same manner as resistor 6, however, the voltage for the series circuit is generated by the rotating armature of motor 10 instead of being supplied by the power source connected to the tracks. That is, when power is removed from a DC motor, it continues to spin because of its inertial mass. The armature, commutator and permanent magnets of the motor combine to act as an electrical generator and a generated voltage potential can be measured across its terminals as it spins. The polarity of the voltage, however, is such that a retarding electromotive force (EMF) is developed in motor 10, which slows down the speed of rotation of the rotating armature with a force related to the inverse of the variable resistance. This effect is commonly referred to as "dynamic braking".
One of the major drawbacks of the resistive type of speed controller is that the resistance of the DC motor and the variable resistor should be matched. Without a good match, an insufficient or otherwise inappropriate range of speed control will result. Another drawback is that the power rating of the resistor should vary with the resistance of the motor. In order to overcome these drawbacks and have the speed controller match the characteristics of a particular motor, one must either change the resistive element of the controller or have a different controller for each type of motor. Dealing with motors of different resistance is common in DC powered modeling and particularly in the "Slot Car" racing hobby. However, this is an undesirable situation. To become proficient at accurately controlling the speed of the model, you must develop a good "feel" for the controller/model combination. If you must be constantly switching among different controllers, or changing the resistive element depending upon the current motor in your model or the current model you happen to be using, it will be very difficult to develop a consistent "feel" for the controller or for the controller/model combination.
Furthermore, since the amount of speed control, as well as the braking, which one achieves with a given amount of wiper movement is non-linear, the "feel" of the speed controller will change, sometimes substantially, with a change in resistive element or motor. For example, for one resistive element you may have the top 50% of your speed control achieved by the upper 30% range of your wiper movement, while with a different resistive element in your controller, that same 50% speed control may only be achievable with the upper 10% of your wiper movement. The same situation happens when braking, and is therefore especially disadvantageous, since it results in applying a difficult to control non-linear braking effect to the model. This type of braking reduces the speed of the model in a way that is relatively difficult to control, thereby making it extremely difficult to developing a winning "feel" for controlling your model.
Additionally, since the resistance of a motor changes with its loading, resistive type speed controllers allow undesirable load responsive speed fluctuations (i.e., due to the loading increasing when the model is directed up an incline), making it even more difficult to develop a consistent "feel" for the controller.
A still further drawback of current speed controllers is the inability to predictably and accurately control a "release" of the braking function (deceleration, then coast), useful, for example, when controlling the model to rapidly negotiate tight turns.
It is an object of the present invention to provide a motor speed/brake controller which overcomes all of the above-discussed drawbacks in a manner which is simple, cost effective and reliable.